This invention relates generally to a method and apparatus for fabricating a semiconductor device, and more specifically to a method and apparatus for plastic encapsulation of high lead count semiconductor devices.
Semiconductor devices are produced in die form and the die is then attached to a lead frame and a protective plastic body is molded about the die. In the case of fabricating a semiconductor device of the type known as a dual inline package (DIP), plastic quad flat pack (PQFP), or the like, the semiconductor die is attached to a die bond area of a metal lead frame and the lead fingers of the lead frame are attached to bonding pads on the die by fine wires, tape, or the like.
A lead frame having many semiconductor dice attached is placed in a mold assembly and a plastic encapsulant material is forced into the mold assembly under high pressure and at an elevated temperature to form plastic bodies about the die, die bond area, inner ends of the lead fingers, and the interconnecting wires or tapes. Because the encapsulating material enters the mold under high pressure, the mold must be carefully sealed to avoid the unwanted escape of material in the formation of "flash", which is a thin extrusion of the encapsulating material attached to the lead frame outside the package body which must be removed.
Usually, an upper mold half and a lower mold half forming a plurality of cavities therebetween is used. A semiconductor devices is located on the lower mold hair mold plate, the upper mold half mold plate is closed on the lead frame and encapsulating material is injected into the cavity to encapsulate the semiconductor device. The mold plates are machined to very tight tolerance. However, it is more difficult to maintain the semiconductor device to the same tight tolerances. The mold plates comprise a rigid material such as tool steel. The semiconductor lead frame is clamped between the mold plates with the intention that the package dimensions be entirely determined by the dimensions of the mold cavity plate. The rigidity of the cavity plate required use of the dam bar to prevent encapsulant from escaping the cavity in the space between the leads. Even when a dam bar was used, however, it is extremely difficult to maintain machining tolerances at a level that will avoid mold flash from escaping through small spaces in the machine surfaces.
The lead frames of complex integrated circuits include many closely spaced metal fingers. To seal the mold and prevent the escape of plastic encapsulating material between the lead fingers, common practice has been to provide a dam bar extending between the leads as disclosed in U.S. Pat. No. 3,444,441. After encapsulation, the dam bar is removed between adjacent lead fingers before device testing. As the number of leads increases and as the spacing between the leads decreases, it becomes more and more difficult to remove the dam bar without damaging or bending the lead fingers. Dam bar removal is expensive. Each different package body and different lead pitch for the same package body requires different dam bar removal tooling. The dam bar removal tool is easily damaged resulting in delays and increased manufacturing cost.
In an alternate process, which is even more expensive, edges of the mold are castellated to provide mold projections which extend between the led fingers. Again, as the number of lead fingers increases and the spacing between them decreases, it becomes extremely difficult and expensive to form and maintain the castellations.
Accordingly, it is an object of the present invention to provide a mold assembly which uses a resilient material to compensate for dimensional variations of both a cavity plate and a semiconductor lead frame being encapsulated.
Another object of the present invention is to use a resilient material to seal a space between leads and replace a conventional dam bar.
Another object of the present invention is to provide a mold apparatus using resilient seals which maintains cavity dimensions determined by dimensions of a rigid cavity plate.
A further object of the present invention is to provide a method for encapsulating a semiconductor devices wherein a sheet of elastic material is used as both a supplementary seal to a clamped cavity plate and a primary seal between leads where no cavity plate extends.